Despite technological advances in the development of alternative fuels (e.g., nuclear and solar) and the use of hydroelectricity (where geography permits), fossil fuels remain the predominant sources of energy today—especially in the automotive market. The major disadvantages of fuels such as gasoline and diesel are that the available supply is limited and that they release significant amounts of waste products, create health hazards, and damage the ozone layer. For years scientists have sought practical alternative sources of power—and most recently fuel cells have shown significant promise. Although initially discovered in 1839, the fuel cell as a potential, viable power source for commercial, residential, and military applications is a relatively recent development, with the majority of significant advances being made over the past two decades.
As a result, there is considerable optimism that the fuel cell may well be the power source of the future, as it runs relatively cleanly, efficiently, and quietly—especially compared to conventional fossil fuel burning combustion sources (such as internal combustion engines). Fuel cells are devices that electrochemically convert a fuel's chemical energy directly to electrical energy—as opposed to power sources that convert chemical energy to mechanical energy using combustion, then convert mechanical energy to electrical energy using a generator. Among the various types of fuel cells available, the Proton Exchange Membrane fuel cell (PEMFC) appears to have the most promise for commercial vehicle and military use, though recent strides in Direct Methanol Fuel Cells (DMFC), and Solid Oxide Fuel Cells (SOFC) have made them more suitable for niche applications.
A typical PEMFC fuel cell is a device that consists of a proton-permeable electrolyte membrane (often Nafion™, though other membranes are available and being researched) sandwiched between two catalyst-impregnated electrodes (an anode and a cathode). A fuel, such as hydrogen, is supplied to the anode side, where the catalyst strips the electrons from the hydrogen atom (leaving a proton). The electrons flow through an external circuit with a load, producing electricity, while the protons pass through the membrane selectively to the cathode, where it recombines with the electrons and oxygen (typically from ambient air) to produce water and heat as byproducts. Different fuel cell types vary in the types of fuel, the membrane/electrolytes, and operating temperature, but virtually all require oxygen (typically from ambient air) at the cathode side. Some fuel cells use hydrogen directly, while others reform secondary hydrocarbon fuels, such as methanol, ethanol, gasoline, diesel, or natural gas, to produce hydrogen—though these reformers are less efficient and pose significant purification issues to remove contaminants from the hydrogen stream that are harmful to the anode-side of the fuel cell.
It is widely known that atmospheric air contains numerous contaminants, both particulate and chemical. These contaminants may range from large items (e.g., leaves, papers, debris, insects, and horticultural products), to small particulate items down to sub-micron sizes (e.g., dust, pet dander, viruses, spores, pollen, smokes, smog, and aerosols). Chemical contaminants are also widely present in atmospheric air, with sources that range from man-made pollution to products of natural environmental causes. Typical atmospheric contaminants include volatile organic compounds such as methane, butane, propane, and other hydrocarbons, oxides of nitrogen, oxides of sulfur, carbon monoxide, hydrogen sulfide, toluene, formaldehyde, etc. In addition, on the military battlefield a whole range of additional contaminants are prevalent—including carbon dioxide, nitrogen, aluminum oxide, graphite flakes, white/red phosphorous, brass flakes, chemical agents, hydrogen cyanide, and so on.
Traditionally, most of today's internal combustion engines are relatively immune to the effect of these contaminants, with the exception of the large particulates (e.g., leaves, debris, dust), which are easily eliminated by relatively simple air filters (most often simple pleated cloth or metal filters). By nature of their design, extremely fine particles and chemical contaminants are handled easily because of the extreme temperatures during combustion. Although combustion of the chemical contaminants may increase emissions, the internal combustion engines performance is not significantly impacted by their presence in the intake air.
Such is not necessarily the case, however, with fuel cells. Fuel cells by their very nature require a relatively clean oxygen source, typically from ambient atmospheric air. Contaminants in the intake (cathode) air stream can, and have been shown to, significantly degrade the cathode catalyst performance, resulting in poor performance, or even preventing operation of the fuel cell.
Only recently has the importance of clean intake air been recognized, though not universally accepted, as a significant drawback to fuel cells. This has been due, in part, to the fact that most research to date has been focused on the effect of impurities in the fuel source on the performance of the PEMFC. Generally these impurities interfere with the catalysts (e.g., platinum) used to accelerate the reaction. However, very little research has been done on examining the effects of cathode oxygen/air impurities on fuel cell performance. Of those studies that exist only a few chemicals, such as carbon monoxide, nitrogen dioxide, sulfur dioxide, and benzene have been examined or characterized. In experiments conducted last year, it was shown that automotive exhaust pollutants (diesel and gasoline) cause a significant degradation in electrical performance and life expectancy of small PEM Fuel Cells, as much as 0.4 to 8.8% per minute. Also it was demonstrated that a simple filter could reduce the degradation by 26 to 85%—which is clearly still too high to make PEMFCs a practical power source. Moreover, it was demonstrated that multiple cycling of dirty versus clean air resulted in a steady degradation of performance, permanently damaging the fuel cell over time. Clearly, the importance of contaminant free cathode intake air has not been fully recognized for its importance on efficient and optimal operation of fuel cells.
Consequently many fuel cells today are not designed to operate efficiently in the presence of large amounts of contaminants which may be present in the intake air that is necessary for the functioning of the fuel cell. They also have not generally been designed to handle or filter such complex contaminant mixes from the intake air. This probably because fuel cells are relatively new, have been tested primarily in relatively benign laboratory environments (where contaminants are limited), and have not been extensively employed in a variety of realistic environments. Moreover, although significant research has (and is being) done on fuel cells, they are still relatively new, and the full scope of their operational parameters are not well defined or understood.
Development of PEMFCs as a viable power source requires a much more advanced air contaminant control system able to deal with wide range of pollutants found in a variety of ambient air environments—in urban areas, rural farming communities, and in military combat. It is well known that atmospheric contamination concentrations vary widely from place to place. The effects of these contaminant concentration variations are particularly acute in mobile applications, such as cars/trucks, small portable fuel cells, auxiliary power units, where the types of and concentration levels of contaminants may vary significantly. It is critical therefore, that fuel cell systems include a highly flexible, adjustable filtration system that is designed to eliminate harmful contaminants and one that enables the fuel cell to be used and easily adapted to a wide range of operational environments. While some limited work has been done in developing simple air filtration systems for specific applications, little has been done to develop a filtration system (or air contamination control system) that can be easily adapted and re-configured during operation to support multiple contaminant environments.
What is desired, therefore, is a fuel cell that functions within environments having a wide range of contaminants, employing easily, relatively lost cost, replaceable cartridges that are tailored in size and amount of filtration media to remove specific particulate and chemical contaminants.